The present invention generally relates to an atrial defibrillator and method for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a system and method for use in an implantable atrial defibrillator for reducing false positives in the detection of atrial fibrillation. More specifically, the system and method of the present invention contemplates qualifying and accumulating time periods between cardiac events detected in an atrial channel during a cardiac signal acquisition period to determine if there is an absence of atrial fibrillation. If there is a failure to detect an absence of atrial fibrillation, an atrial fibrillation detector detects for atrial fibrillation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
In order for an implantable atrial defibrillator to be truly automatic, it must include atrial fibrillation detection to determine if the atria are in fibrillation responsive to monitored activity of the heart. Atrial fibrillation detection must be both sensitive and specific. It must be sensitive so as to not miss an atrial fibrillation episode and specific so as to not misdiagnose a non-atrial fibrillation rhythm as atrial fibrillation. The latter mentioned misdiagnosis is referred to in the art as a false positive, and results in the patient receiving a cardioverting shock when such a shock is not needed.
False positives can impose undesirable consequences. If cardioverting energy is delivered when it is not needed, the patient obviously will experience unnecessary discomfort and inconvenience. Further, such unnecessary cardioverting energy deliveries will accelerate the depletion of the defibrillator battery and hence require early replacement of the defibrillator.
False positives have not been of much concern in the art because the only commercial defibrillators to date have been ventricular defibrillators for treating ventricular fibrillation. Ventricular fibrillation is a life threatening arrhythmia and, as a result, it is generally considered best to tolerate a low percentage of false positives and, to be on the safe side, in providing intervention.
Since the most common rhythm that occurs during the detection for atrial fibrillation is normal sinus rhythm, false positives may be reduced by employing atrial fibrillation detection which is specific to normal sinus rhythm, rejecting this rhythm as not atrial fibrillation. Also, since cardiac activity is generally disorganized and at a high rate during atrial fibrillation, false positives may be further reduced by employing atrial fibrillation detection which is also specific to organized rhythms of relatively low rate. To assure that an atrial fibrillation episode is not mistaken for a non-atrial fibrillation episode, the atrial fibrillation detection should also preferably be sensitive to atrial fibrillation. With this combination, effective atrial fibrillation detection with minimized false positives may be achieved.
Hence, there is a need in the art for an improved atrial fibrillation detection system and method which reduces the potential for false positives in detection of atrial fibrillation. The present invention provides such an improved system and method for use in atrial fibrillation detection which not only is specific to non-potential atrial fibrillation, but which is also sensitive to atrial fibrillation.